My present invention relates to a method of making multiwall, preferably double wall, plastic bags and especially tie bags, i.e. bags having a strip which can be pulled to close the mouth of the bag. More particularly, the invention relates to a method wherein a continuous foil web is fed from an unwinding station and is constituted from thermoplastic synthetic resin.
German patent document DE 44 33 582 describes the production of a double-wall plastic bag in which two flat foil webs are drawn from respective rolls at unwinding stations, the foil webs are superposed so that they are coextensive, thereby forming a double layer web. The double layer web is then folded into a kind of U and at an open head region, grip holes are formed by a stamping or punching process and between the outer layer and the inner layer a pull strip is inserted to form the tie of the bag. The layers of the web are then welded together at the head region in pairs and lateral weld seams can then be provided and the web with the bags formed therein can be perforated to allow separation of the individual bags from one another and the web.
A drawback of this process is that the web on the roll and supplied by the roll for each layer must be twice the height or length, depending upon the orientation of the bag in the web, of the finished plastic bag. It is also known to provide instead of two wide foil webs which are superposed, a flattened foil tube which, like the flat webs, is folded substantially midway along a longitudinal axis into a U shape. The further steps like forming holes serving as hand grips, inserting the tie and the like are practiced more or less as has been described with respect to the flat webs.
Whether one uses two parallel flat webs or a closed tubular web which is flattened and which has a comparatively large web width of the flattened webs, difficulties can be encountered in fabrication and handling of the webs. For example, the use of 2000 mm wide flat foil webs or a 2000 mm wide flattened foil tube for the production of double-layer plastic bags of 1000 mm in length present problems in fabrication. Such widths are difficult to be extruded, the extrusion apparatus required can be unduly costly and because the thickness can be 6 xcexcmm to 8 xcexcmm, such wide foils can only be handled and guided with difficulty. The earlier fabrication systems have had, therefore, only limited production speeds.
It is, therefore, the principal object of the present invention to provide an improved method of making plastic bags, namely multilayer and preferably double-wall plastic bags, especially tie bags, whereby the aforedescribed drawbacks are avoided.
Another object of the invention is to provide a method for the purposes described that simplifies the handling of the foils in the production of such bags and makes such handling problem-free.
Still another object of the invention is to provide a method of making multiwall and preferably double-wall plastic bags which is simplified by comparison with earlier methods and allows the highest possible quality of such bags to be produced at comparatively high production speeds.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in that a plurality, especially two, thermoplastic synthetic resin foil webs are supplied in respective double-layer webs by respective supply rolls and are unwound from the respective rolls at respective unwinding stations, are unfolded to single layer webs of the full web width from the half-width double-layer webs and are then superposed and fed to the bag-shaping stations at which the double-layer full width web is folded for forming the bags therein.
The advantage of the method of the invention is that in the extrusion process in producing the film, a smaller apparatus can be employed with reduced capital cost, greater reliability and a greater ability-to maintain the-uniformity of the extruded foils. A foil tube of a width, of, for example, 1000 mm is more easily extruded than, for example, a foil tube of a width of 2000 mm.
When the double-layer foil web is in the form of a flattened tube or tubular web, it can be split by a central longitudinal cut along one of the wall sides and the resulting flaps can be folded outwardly. The remaining walls of the tube and the two downwardly folded flaps are thus spread to form the full width foil which is superposed in a second full width foil made by slitting the other foil tube. The central slitting reduces the width of the flaps which must be unfolded by half in comparison to the width which would have to be unfolded if the slit would be provided along one of the edges of the flattened tube.
In addition, the central slit forms perfectly straight edges for the full width foil web, thereby greatly simplifying the superimposition of the two full width foil webs and the subsequent folding and processing to form the bags.
Advantageously, the tubular webs are supplied in the travel direction of the unfolded flattened full width webs with the foil webs being correspondingly deflected.
The tubular webs can also be fed substantially transversely to the travel direction of the unfolded foil webs and likewise guided over bars or rollers at which direction changes take place.
It is thus possible to unfold a plurality of half tubular or semi-tubular webs to form unfolded flattened members which are superposed and then folded into a kind of U to fabricate the plastic bags in a bag-making machine. In that case, the semi-tubular members may comprise two superposed walls which unitarily are connected along at least one edge and are unfolded without the need for slitting.
The method of the invention thus comprises the steps of:
(a) unwinding a plurality of double-layer substantially half-width foil webs from respective rolls and guiding the unwound double-layer substantially half-width foil webs along initial segments of a path;
(b) unfolding the double-layer substantially half-width foil webs along the initial segments into substantially full-width single-layer webs;
(c) superimposing one of the substantially full-width single-layer webs on at least one other of the substantially full-width single-layer webs coextensively to form a multilayer full-width web along a subsequent segment of the path;
(d) folding the multilayer full-width web generally in a U shape along a further segment of the path to form a folded multilayer web; and
(e) forming bags in the folded multilayer web.